Some wireless communications systems provide users with multiple types of services. For example, a particular system might provide voice, paging, and other services to its users.
The data and communications channels required for various types of services often have different characteristics. For example, voice data and paging data are generated at different bit rates, and voice and data channels may have different transmit link margin requirements. A voice data rate for a single channel might use a bandwidth of 50 kilohertz (kHz) and require a high link margin to ensure good quality service. On the other hand, paging data is more bursty in nature, and it might occur at a data rate that is tenths or hundredths the rate of voice data. In addition, a paging channel typically requires a substantially higher link margin than a voice channel, to ensure that the paging signals are likely to be received when the user is in a building or in the presence of other obstructions.
In order to provide services with different bit rates and link margin requirements, some prior art systems allocate separate spectral bands to each type of service. FIG. 1 illustrates an example of a spectral allocation diagram in accordance with the prior art. The example spectrum includes six frequencies 101, 102, 103, 104, 105, and 106. At each frequency 101-106, data can be transmitted in one of four timeslots 111, 112, 113, and 114. Accordingly, the system uses a combination of FDMA and TDMA to transmit data on different channels.
Because in some systems the paging data channels have a higher link margin requirement than the voice channels, such systems increase the transmission power when transmitting paging data. Accordingly, it is necessary to allocate separate timeslots and frequencies to the voice data and the paging data. This is shown in FIG. 1, where the voice channels 120 are allocated to timeslots 111-113 and frequencies 101-104. The paging channels 130 are allocated timeslot 114 and frequencies 105 and 106. The transmission power is increased by devoting all the carrier energy to a single one of two frequencies 105 or 106 in time slot 114 to meet the higher paging link margin requirements, during which time slot the carrier energy is not available for transmitting on the other carrier frequencies 101-104.
Because the voice and paging channels do not overlap, a substantial amount of the spectrum, represented by cross-hatched area 140, is not useable. This technique of separating the voice and paging data in frequency and time results in an inefficient use of available spectrum. In addition, only a single paging channel can be placed in each of the available frequencies. Thus, each of the relatively low rate paging data channels uses substantially more spectrum than its data rate warrants.
What are needed are methods and apparatus for a communications system to provide multiple wireless communications services, each of which has a different bit rate and/or link margin requirement. Further needed are methods and apparatus for providing different bit rate services, where available spectrum is used in an efficient manner. Additionally needed are methods and apparatus that use a common transmission bit rate, thus potentially simplifying the design of transmit and receive equipment.